Heretofore, as a display element which the display device includes, there are: an electro-optical element in which brightness is controlled by a voltage applied thereto; and an electro-optical element in which brightness is controlled by a current flowing therethrough. As a representative example of the electro-optical element in which the brightness is controlled by the voltage applied thereto, a liquid crystal display element is mentioned. Meanwhile, as a representative example of the electro-optical element in which the brightness is controlled by the current flowing therethrough, an organic EL element is mentioned. The organic EL element is also referred to as an OLED (Organic Light-Emitting Diode). In comparison with the liquid crystal display device that requires a backlight, color filters and the like, an organic EL display device using the organic EL element that is a light emission-type electro-optical element can easily achieve thinning, reduction of electric power consumption, enhancement of the brightness, and the like. Hence, in recent years, development of the organic EL display device has been progressed positively.
As a drive method for the organic EL display device, a passive matrix method (also referred to as a simple matrix method) and an active matrix method are known. An organic EL display device that adopts the passive matrix method has a simple structure; however, a size increase and definition enhancement thereof are difficult. In contrast, an organic EL display device that adopts the active matrix method (hereinafter, referred to as an “active matrix-type organic EL display device”) can easily realize the size increase and the definition enhancement in comparison with the organic EL display device that adopts the passive matrix method.
In the active matrix-type organic EL display device, a plurality of pixel circuits is formed in a matrix fashion. Typically, each of the pixel circuits of the active matrix-type organic EL display device includes: an input transistor that selects a pixel; and a drive transistor that controls supply of a current to the organic EL element. Note that, in the following, the current flowing from the drive transistor to the organic EL element is sometimes referred to as a “drive current”.
FIG. 37 is a circuit diagram showing a configuration of a conventional general pixel circuit 91. This pixel circuit 91 is provided so as to correspond to each of crossing points of a plurality of data lines S and a plurality of scanning lines G, which are arranged on a display unit. As shown in FIG. 37, this pixel circuit 91 includes: two transistors T1 and T2; one capacitor Cst; and one organic EL element OLED. The transistor T1 is an input transistor, and the transistor T2 is a drive transistor.
The transistor T1 is provided between the data line S and a gate terminal of the transistor T2. With regard to the transistor T1, a gate terminal thereof is connected to the scanning line G, and a source terminal thereof is connected to the data line S. The transistor T2 is provided in series to the organic EL element OLED. With regard to the transistor T2, a drain terminal thereof is connected to a power supply line that supplies a high-level power supply voltage ELVDD, and a source terminal thereof is connected to an anode terminal of the organic EL element OLED. Note that the power supply line that supplies the high-level power supply voltage ELVDD is hereinafter referred to as a “high-level power supply line”, and the high-level power supply line is denoted by the same reference symbol ELVDD as that of the high-level power supply voltage. With regard to the capacitor Cst, one end thereof is connected to the gate terminal of the transistor T2, and other end thereof is connected to the source terminal of the transistor T2. A cathode terminal of the organic EL element OLED is connected to a power supply line that supplies a low-level power supply voltage ELVSS. Note that the power supply line that supplies the low-level power supply voltage ELVSS is hereinafter referred to as a “low-level power supply line”, and the low-level power supply line is denoted by the same reference symbol ELVSS as that of the low-level power supply voltage. Moreover, here, a connecting point of the gate terminal of the transistor T2, the one end of the capacitor Cst and the drain terminal of the transistor T1 is referred to as a “gate node VG” for the sake of convenience. Note that, in general, either one of the drain and the source, which has a higher potential, is referred to as the drain. However, in the explanation of this description, one thereof is defined as the drain, and the other thereof is defined as the source. Accordingly, in some case, a source potential becomes higher than a drain potential.
FIG. 38 is a timing chart for explaining operations of the pixel circuit 91 shown in FIG. 37. Before a time t1, the scanning line G is in a non-selection state. Hence, before the time t1, the transistor T1 is in an OFF state, and a potential of the gate node VG maintains an initial level (for example, a level corresponding to a write in an immediately previous frame). When the time t1 comes, the scanning line G turns to a selection state, and the transistor T1 turns ON. Thus, a data voltage Vdata corresponding to brightness of a pixel (sub-pixel), which is formed by this pixel circuit 91, is supplied to the gate node VG via the data line S and the transistor T1. Thereafter, during a period until a time t2, the potential of the gate node VG changes in response to the data voltage Vdata. At this time, the capacitor Cst is charged with a gate-source voltage Vgs that is a difference between the potential of the gate node Vg and the source potential of the transistor T2. When the time t2 comes, the scanning line G turns to the non-selection state. Thus, the transistor T1 turns OFF, and the gate-source voltage Vgs held by the capacitor Cst is determined. The transistor T2 supplies a drive current to the organic EL element OLED in response to the gate-source voltage Vgs held by the capacitor Cst. As a result, the organic EL element OLED emits light with brightness corresponding to the drive current.
Incidentally, in the organic EL display device, typically, a thin film transistor (TFT) is adopted as the drive transistor. However, the thin film transistor is prone to cause variations in a threshold voltage. When the variations of the threshold voltage occur in the drive transistor provided in the display unit, variations of the brightness occur, and accordingly, display quality is decreased. Accordingly, heretofore, there has been proposed a technology for suppressing the decrease of the display quality in the organic EL display device. For example, Japanese Patent Application Laid-Open No. 2005-31630 discloses a technology for compensating the variations of the threshold voltage of the drive transistor. Moreover, Japanese Patent Application Laid-Open No. 2003-195810 and Japanese Patent Application Laid-Open No. 2007-128103 disclose a technology for constantly maintaining a current flowing from the pixel circuit to the organic EL element OLED. Furthermore, Japanese Patent Application Laid-Open No. 2007-233326 discloses a technology for displaying an image with uniform brightness irrespective of the threshold voltage and electron mobility of the drive transistor.
In accordance with the above-mentioned prior arts, even when the variations of the threshold voltage occur in the drive transistor provided in the display unit, it becomes possible to supply a constant current to the organic EL element (light-emitting element) in response to desired brightness (target brightness). However, with regard to the organic EL element, current efficiency thereof is decreased with the elapse of time. That is to say, even when the constant current is supplied to the organic EL element, the brightness is gradually decreased with the elapse of time. As a result, the burn-in occurs.
As stated above, if no compensation is made for such a deterioration of the drive transistor and such a deterioration of the organic EL element, then as shown in FIG. 39, a current decrease resulting from the deterioration of the drive transistor occurs, and in addition, a brightness decrease resulting from the deterioration of the organic EL element occurs. Moreover, even if the compensation is made for the deterioration of the drive transistor, the brightness decrease resulting from the deterioration of the organic EL element occurs as the time elapses as shown in FIG. 40. So, Japanese Unexamined Patent Application Publication No. 2008-523448 discloses a technology for making correction for data based on characteristics of the organic EL element OLED in addition to a technology for making correction for the data based on characteristics of the drive transistor.